I. Field of the Invention
The present invention relates generally to the field of cardiac resynchronization therapy (CRT), and more specifically, to a physiologic method for determining optimal atrioventricular (AV) delay values over a range of heart rates from rest to the upper tracking or paced heart rate (HR) and for determining the sensed to paced AV delay offset for optimizing the patient outcome from such therapy. The disclosed method enables physicians to improve the process of programming biventricular pacemakers/ICDs and DDDR pacemakers.
II. Related Art
Several pacemaker manufacturers have the capability of programming a dynamic or auto-decrementing AV delay which decreases linearly from the resting programmed heart rate to the upper tracking or paced HR. The decrement range is not physiologically based upon required left ventricle (LV) filling times or a mid-point AV delay between the lower and upper pacemaker rates which is shown to be the most effective in terms of combined cardiac pump function and breathing efficiency.
The status of the clinical use of a dynamic AV delay is that it has been met with reluctance by the follow-up physician since there is presently no way to assign an adequate resting AV delay. All too often, the dynamic AV delay is set too aggressively based on the use of a linear function which shortens the time at the upper rate excessively. In doing so, ventricular filling and stroke volume output of the heart is compromised, and gas exchange in the lungs and breathing efficiency are adversely affected. The scientific literature has noted that AV conduction times vary inversely to fluctuation of atrial excitement rhythm (Beat-to-beat Modulation of Atrioventricular Conduction During Dynamic Exercise in Humans, Nakamoto, et al, Japanese Journal of Physiology Vol. 55, 37-51, 2005). The cited article also describes this relationship as curvilinear rather than straight line.
In addition to the above, there is no physiologic method other than a pacemaker programmer and an electrically based intra-cardiac electrogram (IECG) technique to estimate the correct AV sensed to paced offset for a cardiac resynchronization therapy (CRT) patient experiencing both sensed and paced atrial activity via the device's data logger. The electrophysiologist typically guesses at whether the optimal AV sensed/paced offset is 30 msec, 45 msec or 50 to 60 msec. It is known that paced conduction time to the left ventricle takes longer since the conduction pulse travels more slowly through atrial/ventricular muscle tissue vs. specialized conducting tissue, as with a paced atrial beat from the atrial appendage or lateral RA vs. an intrinsic atrial beat originating from the sinus node.
Regarding the programming of the upper tracking or paced HR, the only means to select the HR is by “x” percent of the patient's age predicted max HR, the level of patient activity or whether they have ischemic heart disease. Upper tracking or paced HR can range from 110 to 150 bpm, depending on the above criteria and other programmed timing intervals such as the post ventricular atrial refractory period (PVARP).
U.S. Pat. No. 7,225,022, “Method for Optimizing Patient Outcome from Cardiac Resynchronization Therapy”, of common inventorship as the present application, discloses a method for determining the optimal AV and VV delay interval using “variables indicative of one or more functions selected from the group consisting of forward pump function (stroke volume output) and retrograde effects of filing pressures, pulmonary venous flow, and gas exchange at the alveolar/capillary membrane interface during exercise”. That patent also discloses a method in which a single set of equipment is utilized to optimize all phases/aspects of cardiac resynchronization therapy, including appropriate rate response during exercise/activity and device programming, including dynamic AV and VV delay of which resting AV and VV delay are a portion. Accordingly, the above-referenced patent is deemed incorporated herein by reference in its entirety for any purpose. In the present application, a novel method for determining dynamic AV delay and AV delay offset is disclosed that may be accomplished using a set of equipment such as that used in the above-referenced patent, and as described in FIG. 1.
Definitions of Terms
The following contains definitions and explanations of certain terms as used in the present context.    Upper tracking or paced HR—The programmed Upper tracking or paced Rate is the highest pacing rate at which ventricular tracking of atrial sensed or paced events can occur (in the DDDR, DDD, and VDD modes).    End-Tidal Partial Pressure of CO2 (PetCO2, ETCO2)—The partial pressure of carbon dioxide at the end of expiration, or the highest value of PCO2 during a single expiration.    Forward Pump Function—Refers to the ability of the heart to contract and eject blood which has returned to the heart during its relaxation, or filling, cycle via the aorta against a given amount of resistance, or after load.    Oxygen Pulse (O2 Pulse)—O2 Pulse is an indirect index of combined cardiopulmonary oxygen transport. It is calculated by dividing oxygen uptake (ml/min) by heart rate. In effect, O2 Pulse is equal to the product of stroke volume and arteriovenous O2 difference. Thus circulatory adjustments that occur during exercise, that is, widening arteriovenous O2 difference, increased cardiac output, and redistribution of blood flow to the working muscle, will increase O2 Pulse. Maximal O2 pulse is higher in fitter subjects, lower in the presence of heart disease, and, more importantly, higher at any given workload in the fitter or healthier individual. On the other hand, O2 Pulse will be reduced in any condition that reduces stroke volume . . . ” V. Froelicher, J. Myers, et al., Exercise and the Heart. Mosby-Year Book, Inc. 1993, p. 38    Retrograde Pump Function—Refers to the filling of the heart during the relaxation part of the cardiac cycle. Filling pressure and the volume of blood that returns to the heart during diastole are termed preload. Any forward pump failure of the heart can increase the preload on the heart to undesirable levels which, in turn, has an adverse retrograde effect on gas exchange in the lung.    Ventilation-Perfusion Coupling—“For gas exchange to be most efficient, there must be a precise match, or coupling, between ventilation (the amount of gas reaching the alveoli) and perfusion (the blood flow in pulmonary capillaries). Changes in the PCO2 within the alveoli cause changes in the diameters of the bronchioles. Passageways servicing areas where alveolar carbon dioxide levels are high dilate, allowing carbon dioxide to be eliminated from the body more rapidly; those servicing areas where the PCO2 is low constrict. As a result of the modifications these two systems (also for PO2), alveolar ventilation and pulmonary perfusion are always attempting to synchronize. Poor alveolar ventilation results in low oxygen and high carbon dioxide levels in the alveoli; consequently, the pulmonary capillaries constrict and the airways dilate, bringing airflow and blood flow into closer physiological match. High oxygen and low carbon dioxide alveolar partial pressures cause constriction of the respiratory passageways and a flushing of blood into the pulmonary capillaries. At all times, these homeostatic mechanisms provide the most appropriate conditions for efficient gas exchange.” E. Marieb, Human Anatomy and Physiology. Benjamin/Cummings Publishing Company, 1992, p. 749    Ventilatory Equivalent for carbon dioxide (VE/VCO2, EQCO2)—The EQCO2 is calculated by dividing ventilation (L/min) by VCO2 (L/min). “VE/VCO2 represents the ventilatory requirement to eliminate a given amount of CO2 produced by the metabolizing tissues. Since metabolic CO2 is a strong stimulus for ventilation during exercise, VE and VCO2 closely mirror one another, and after a drop in early exercise, VE/VCO2 normally does not increase significantly throughout sub-maximal exercise. However, in the presence of chronic heart failure, VE/VCO2 is shifted upward compared to normal, and high VE/VCO2 values are one of the characteristics of the abnormal ventilatory response to exercise in this condition.” Ibid Froehlicher.    Estimate of the Drive to breathe (VT/Ti)—Tidal volume is the volume of an average breath; inspiratory time is the average time it takes to inspire. The ratio has been used as an index of ventilatory drive (the combined stimulation to breathe).